Conventionally, in a communication power supply or emergency power supply, a secondary battery is used as a backup power supply, and in most cases, a lead-acid storage battery has been used as the secondary battery provided for this purpose. Such a lead-acid storage battery has been used for years, in which stable performance is maintained even after extended period of use thereof, so that it is highly reliable. Further, such a lead-acid storage battery can be produced so as to have a large capacity, such as thousands of ampere-hours (Ah), although the output voltage is as low as 2 V. The battery can be changed merely by connecting it to a charger, without providing a special battery protector. A rise in voltage can be prevented by using a water-based electrolytic solution. In particular, when the water-based electrolytic solution is used, electrolysis of the water may start after the charging of the battery is completed, even if the battery is overcharged because of a failure in a rectifier. Accordingly, the maximum value of the terminal voltage is suppressed to the voltage of the electrolysis (about 3 V) or less, and this safety feature prevents a further voltage rise.
The lead-acid storage battery used in power supply systems has features of usability. However, because the battery is composed of mostly an electrode material, which is a lead-based material, the weight and volume of the battery per unit energy are large. Therefore, in order to store the same energy in a lead-acid storage battery, the volume and weight of the battery becomes larger than the others. These problems can be solved by using a secondary battery having less weight and volume per unit energy, or having a greater discharging energy per unit weight or volume.
A secondary battery having a high energy density is a lithium-ion battery. In the lithium-ion battery a lithium and metallic oxide compound is used for the cathode and a carbon-based material is used for the anode. Lithium ions liberated from the cathode and moving between the cathode and the anode causes reactions in a charge and discharge. Lithium ions are taken into the carbon during charging, and during discharging, the lithium ions move to the oxide in the cathode and enter the cathode.
Because of its high energy density, the lithium-ion battery largely contributes to reduction of the size and weight of the system and has widely used in mobile telephone terminals, personal computers, and the like. In such a small apparatus since an operating voltage is low and an output voltage operated at the lithium-ion battery is about 4 V, about one to three lithium-ion batteries connected in series may operate such an apparatus. Thus the lithium-ion batteries are thus useful in reducing the size of the system. Unlike lead-acid storage batteries, in such a lithium-ion batteries, an electrolytic solution whose main component is an organic liquid is used, and therefore, a function for suppressing a rise of terminal voltage during charging does not work. Accordingly, the terminal voltage must be monitored during charging, and a certain measure to prevent the voltage rise must be provided. Discharging must be stopped when the terminal voltage decreases to a predetermined level. This is because copper used as a conductive material serving as an electricity path from the electrode to a load (at the anode) starts liberating ions at the above-mentioned or less voltage. The liberated copper ions become impurities that interfere with the reaction of the battery.
When a battery like a lithium-ion battery is used, the voltage must be monitored to control charging and discharging. The temperature must also be monitored, and some measures, such as stopping the charging of the battery, must be taken when the temperature reaches a certain level. Conventionally, in a mobile phone, charging the lithium-ion battery is controlled in accordance with a monitored temperature or the like (for instance, refer to Kazuhiko Takeno, et al. “Methods of Energy Conservation and Management for Commercial Li-ion Battery Packs of Mobile Phones,” Proceedings of INTELE03, 15-1, p. 310). A special chip for controlling the charging of a lithium-ion battery has also been developed (refer to Symposium on Battery Technology, 2001, 4-2-1, Japan Management Association, for instance).
A battery like a 100 Ah lithium-ion battery for use in electric vehicle has been developed in recent years. A battery pack containing lithium-ion batteries is used with a charging control circuit, thereby controlling charging and discharging or monitoring each cell voltage (refer to GS News Technical Report, Vol. 59, No. 2, p. 23, 2000). As the batteries for use in an electric vehicle are charged and discharged repeatedly, just charge control and discharge management are performed.
Batteries like lithium-ion batteries for use as a backup in a general-purpose power supply have also been produced in recent years. The backup secondary battery in a DC power supply system for supplying power directly to a communication equipment such as a telephone switching is maintained by a floating charge method, and a load device and the secondary battery are connected in parallel on the output side of a rectifier. The rectifier always supplies DC power to the load and supplies a charging current needed to maintain the battery in a fully charged state, in accordance with the charging state of the secondary battery. In this system, the secondary battery discharges in case that any power failure or failure of the rectifier occurs, and the power supply to the load device will thus never be interrupted. This is because the capacity of the secondary battery is selected so that necessary power can be supplied by discharging the secondary battery during a period when the DC power supply system needs the power.
In such a system, in which a high reliability is required when operated with a backup power supply, in order to determine the status of the secondary battery, such as the remaining operatable life span (measured in years), the remaining capacity, and the like, may be used in addition to the controlling charging and discharging. A communication power supply system has a remote monitoring unit for a sealed lead-acid storage battery used as a backup power supply, which measures the internal resistance and the terminal voltage of the secondary battery (refer to Kiyoshi Takahashi and Yuichi Watakabe, “Development of SOH Monitoring System for Industrial VRLA Battery String,” Proceedings of INTELECO3, 35-1, p. 664, for instance).
In one method of keeping track of the status of the backup secondary battery, the power supply system lowers the output voltage of the rectifier for a predetermined period of time while power is being supplied, thereby causing the secondary battery to discharge, in order to check whether the secondary battery supplies normal power (refer to Kazuhiko Takeno, et al., the voltage characteristics based on “Methods of Energy Conservation and Management for Commercial Li-ion Battery Packs of Mobile Phones,” Proceedings of INTELE03, 15-1, p. 310).
In addition, a power converter is known in which its reliability is improved by allowing a single user board to back up power upon a power failure (by supplying a voltage from a lithium battery to a DC-DC module), and the front-end section can be made smaller and lighter (disclosed in Japanese Unexamined Patent Application Publication No. 2004-64978 (Abstract and FIG. 1)).